Recently, mobile electronic equipment, such as a mobile phone, a personal digital assistant (PDA), a notebook personal computer, a digital camera, or a digital camcorder, has become multifunctional. The amount of information processed by this equipment has increased, leading to a larger power consumption.
Therefore, there is a great demand for a higher energy density of a mounted battery.
A fuel cell directly converts chemical energy, which is obtained by chemically reacting hydrogen with oxygen, into electric energy.
An energy density of hydrogen itself is high, and oxygen is taken in from an outside air, so an active material is not necessarily provided in advance to a cathode side. Therefore, an energy capacity per volume/per mass can be dramatically increased as compared to a conventional fuel cell.
Among those, a polymer electrolyte fuel cell (PEFC) is suitably mounted onto a portable electronic apparatus, since it has high handling ability, is actuated at low temperature, can be quickly started/stopped, etc.
The polymer electrolyte fuel cell basically includes a proton conductive polymer electrolyte membrane and a pair of electrodes provided on both sides of the polymer electrolyte membrane.
The electrode includes a catalyst layer made of platinum or a platinum-group metal catalyst and a gas diffusion electrode formed on an outer surface of the catalyst layer to supply and collect a gas.
A member obtained by integrating the electrodes and the polymer electrolyte membrane is called a membrane electrode assembly (MEA). A fuel (hydrogen) is supplied to one of the electrodes, and an oxidizer (oxygen) is supplied to the other of the electrodes, thereby performing power generation.
At this time, water is generated as a product thereof. Reaction formulae in the anode and the cathode are as follows.anode: H2→2H++2e−cathode: ½O2+2H++2e−→H2O
A logical voltage of one membrane electrode assembly set is about 1.23 V. In a normal operation state, the membrane electrode assembly is often used when the logical voltage is about 0.7 V. Therefore, when a higher electromotive voltage is required, a plurality of cell units is stacked and is electrically connected in series for use.
The stacked structure as described above is called a fuel cell stack. Normally, in the stack, an oxidizer flow path and a fuel flow path are separated from each other by a member called a separator.
There are various types of fuels for the fuel cell. There are employed a method of directly supplying a liquid fuel, such as methanol, a method of supplying pure hydrogen, and a method of supplying hydrogen by reforming a liquid fuel.
For the portable electronic apparatus, the hydrogen supplying method is preferable because an output is high and the hydrogen supplying method is advantageous in downsizing.
In the polymer electrolyte fuel cell, in a state where a circuit for connecting output terminals of the fuel cell and a load to each other is in an open state, when the fuel and the oxidizer are left to remain, the fuel causes catalytic combustion due to a phenomenon of the fuel passing through a polymer electrolyte membrane (cross leakage), resulting in deterioration of the fuel cell.
Further, it is known that when the fuel and the oxidizer remain to cause a difference in potential between a fuel electrode and an oxidizer electrode, deterioration of components, such as a catalyst and an electrolyte, is promoted.
In order to prevent the deterioration, at the time of stopping the fuel cell, a unit for quickly eliminating the remaining fuel and oxidizer is required.
In a conventional technique, in order to prevent the deterioration, Japanese Patent Application Laid-Open No. H07-272740 discloses a method of purging an inside of a gas flow path using an inert gas (nitrogen) at a time of stopping the fuel cell.
However, in this method, it is necessary to mount a container in the fuel cell system, for storing the inert gas.
In this case, as a method of obtaining the inert gas, Japanese Patent Application Laid-Open No. 2002-50372 discloses a method of obtaining the inert gas for purging, by reacting hydrogen with air using a hydrogen combustor.
Further, Japanese Patent Application Laid-Open No. H08-255625 discloses a method of reducing a voltage between terminals of the fuel cell to be equal to or lower than a predetermined voltage by consuming a residual gas by power generation of the fuel cell after the purging.
Further, Japanese Patent Application Laid-Open No. 2003-317770 discloses a fuel cell system having a structure in which, after stopping the fuel cell, by shutting off distribution of a fuel gas and short-circuiting the terminals, the fuel in the anode is consumed.
Further, in a large fuel cell system, in many cases, an amount of the fuel supplied and circulated is equal to or larger than an amount of the fuel consumed in power generation. On the other hand, in a fuel cell system for a small electronic apparatus, in many cases, there is employed a system in which the fuel flow path is dead ended, and only an amount of the fuel consumed is supplied.
Note that, in this case, there is a problem in that an impurity is accumulated in the fuel flow path, so that power generation performance degrades over time.
Thus, conventionally, a purge valve is provided in the flow path to periodically perform a purge operation.
In particular, U.S. Pat. No. 6,423,437 discloses a technique in which, in a dead-ended small fuel cell, without using an active purge valve, the fuel flow path is purged by a passive mechanism, thereby preventing degradation of the power generation performance.
A relief valve is also called a safe valve, and is a valve which opens when a pressure in the flow path exceeds a predetermined pressure to release the pressure to the outside.
A specific structure thereof is as illustrated in Japanese Patent Application Laid-Open No. H06-94147.
Meanwhile, a check valve is a valve having a function of allowing a fluid to flow in only one direction and preventing the fluid from flowing in the opposite direction.
A specific structure of the check valve is as illustrated in Japanese Patent Application Laid-Open No. H05-126267.
However, in the above-mentioned conventional technique, provided is a fuel cell system having a structure in which an amount of a gas supplied is equal to or larger than an amount thereof consumed in the power generation, and the supplied gas is circulated in the flow path.
Those include an actively-controlled scavenge unit or discharge unit, so there is a problem of inducing an increase in the size of the system.
Further, in a case where a resistor is connected to the fuel electrode and the oxidizer electrode of the fuel cell to consume a residual gas, when the purging is not performed, a pressure in the fuel flow path or in the oxidizer flow path becomes a negative pressure, thereby causing a pressure difference between the fuel electrode and the oxidizer electrode to fluctuate to a large degree.
Thus, there is a problem in that mechanical deterioration or leakage is promoted.
Conversely, when the purging is performed, there is a problem in that an unreacted fuel is perhaps released to the outside of the system.
Further, in a case where a plurality of fuel cell units is connected in series (stacked) for use, there arises the following problem.
That is, in an operation of the fuel cell in a state close to a power generation performance limit of the fuel cell in which terminals are short-circuited, there is a case where polarity inversion occurs in which one fuel cell unit is forcedly applied with voltage of the other fuel cell units, thereby causing deterioration of the fuel cell.
Further, in a small fuel cell system for a portable electronic apparatus, as an oxidizer gas, oxygen in air is supplied by natural diffusion, and a pressure of hydrogen as the fuel gas is set slightly higher than the atmospheric pressure, thereby allowing the hydrogen to be introduced and diffused in the fuel flow path.
The fuel flow path has a dead-ended mechanism, which is a non-circulation system having an inlet for hydrogen, and an outlet therefor, which is basically closed. In many cases, there is used a system in which only an amount of the fuel consumed is charged.
Further, in order to reduce the size, it is desirable to omit as many unnecessary accessories as possible.
Therefore, it is desirable that by replacing air in the fuel flow path instead of introducing an inert gas, generation of a difference in potential between the fuel electrode, which is an anode, and the oxidizer electrode, which is a cathode, be prevented.
In this case, an autonomous control needs to be performed without using a controller for controlling scavenging and discharging, thereby achieving both quick air replacement in the fuel flow path and prevention the release of an unreacted fuel to the outside of the system.
Further, a structure with which the pressure difference between the anode and the cathode can be quickly eliminated is required. The technique disclosed in the above-mentioned U.S. Pat. No. 6,423,437 does not fully satisfy those requirements.